Use of additives in electrolyte for electrochemical cells

ABSTRACT

The invention relates to the use of salt-based compounds as additives in electrolytes for improvinq the properties of electrochemical cells.

BACKGROUND

The invention relates to the use of salt-based compounds as additives inelectrolytes for improving the properties of electrochemical cells.

Lithium ion batteries are amongst the most promising systems for mobileapplications. The areas of application extend from high-qualityelectronic equipment (for example mobile telephones, camcorders) tobatteries for electrically driven vehicles.

These batteries consist of a cathode, an anode, a separator and anon-aqueous electrolyte. The cathode is typically Li (MnMe_(z))₂O₄,Li(CoMe_(z))O₂, Li (CoNi_(x)Me_(z))O₂ or other lithium intercalation andinsertion compounds. Anodes can consist of-lithium metal, carbon,graphite, graphitic carbon or other lithium intercalation and insertioncompounds or alloy compounds. The electrolyte can be a solutioncontaining lithium salts, such as LiPF₆, LiBF₄, LiClO₄, LiAsF₆,LiCF₃SO₃, LiN(CF₃SO₂)₂ or LiC(CF₃SO₂)₃ and mixtures thereof, in aproticsolvents.

A multiplicity of additives for use in lithium ion batteries ismentioned in the literature. For example, in EP 0759641 and U.S. Pat.No. 5,776,627, organic aromatic compounds, such as bi-phenyl,substituted thiophenes and furans, and in EP 0746050 and EP 0851524,substituted anisole, mesitylene and xylene derivatives are added to theelectrolyte in order to increase the safety of the battery in the caseof, overcharging. For the same purpose, U.S. Pat. No. 5,753,389 usesorganic carbonates as additives In order to improve the cycle stability,organic boroxines are added in EP 0856901. However, all these additiveshave some crucial disadvantages. Organic substances, as used in thespecifications mentioned here, generally have low flash points and lowexplosion limits.

Additive Explosion limit [%] Flash point [° C.] Thiophene 1.5-12  −9Anisole 0.34-6.3  43 Mesitylene 1-6 54 Furan  2.3-14.3 −35

Since the use of electrochemical cells and in particular the occurrenceof faults (for example short-circuiting, mechanical damage) is alwaysaccompanied by warming, escape of the electrolyte represents anadditional source of danger.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide additiveswhose volatility is low and whose flash points are relatively high. Thisobject according to the invention is achieved by the use of organicalkali metal or tetraalkylammonium salts as additive.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

The organic alkali metal salts are dissolved in electrolytes which areusually employed in non-aqueous secondary lithium batteries.

It has been found that the additives participate in formation of thecoating layer on the anode and cathode. The coating layer results inpassivation of the electrodes and thus in an increase in the cyclabilityof the electrodes. Film formation on the cathode can in addition serveincrease safety in the event of overcharging, since after release of amechanical safety means, for example by a disconnector, as described inU.S. Pat. No. 5,741,606, the voltage is dissipated by “internalself-discharge”.

The additives ace distinguished by very high thermal decompositionpoints. A crucial advantage over the additives used hitherto is theformation of a glass-like, polymeric layer on thermal decomposition,which can be caused, for example, by a short-circuit.

The invention therefore relates to an electrolyte for non-aqueoussecondary lithium batteries which improves the performance, such as, forexample, the coating layer formation, cyclability, safety, conductivityand low-temperature behaviour, through the addition of specificadditives.

Surprisingly, it has been found that lithium salts which activelyparticipate in the formation of a passivating coating layer on thegraphite electrode are suitable for improving the passivation of theanode. It has been found that the quality of the coating layer iscrucially improved. Reduction of the additive gives a film which ispermeable to lithium ions on the anode. This film leads to improvedcyclability of the anode from only the second cycle.

It has furthermore been found that these additives decompose oxidativelyat potentials above the charge potential of the selected cathode andthus form a passivating film on the cathode. These films are permeableto lithium ions and protect the selected solvent and conductive saltagainst oxidative decomposition.

Use in battery systems based on LiCoO₂ and LiNiO₂ appears particularlyinteresting. It is known that these electrode materials are unstable inthe overcharged state. This can result in a vigorous reaction with theelectrolyte, with corresponding safety risks occurring. The state of theart consists of internal safety mechanisms, such as, for example,so-called disconnectors. On overcharging of a battery, gaseouscomponents are generally liberated, with evolution of heat. Theresultant pressure increase results in the disconnector breaking thecontact between electrode and current conductor and thus preventingfurther overcharging of the battery. A problem here is that the batteryremains in the charged, unstable state.

External discharge is no longer possible owing to the irreversiblebreaking of contact.

The aim is, through addition of selected additives, to apply a film tothe cathode in the event of overcharging, i.e. at potentials above thecharge voltage, which film reacts with the cathode in a controlledmanner after addressing of the disconnector and thus dissipates the“excess potential” through internal self-discharge.

A general example of the invention is explained in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a voltammogram for the experiment of Example 1;

FIG. 2 shows a voltammogram for the experiment of Example 1;

FIG. 3 shows a voltammogram for the experiment of Example 1;

FIG. 4 shows a voltammogram for the experiment of Example 2a;

FIG. 5 shows a voltammogram for the experiment of Example 2b;

FIG. 6 depicts the experiment of Example 4, showing acharging-discharging curve; and

FIG. 7 depicts the experiment of Example 4, showing acharging-discharging curve.

DETAILED DESCRIPTION

a) Behaviour of the Additives at Low Potentials

In each case, 3-5 cyclic voltammograms are recorded successively in ameasurement cell containing an electrode of lithium metal, carbon,graphite, graphitic carbon or other lithium intercalation and insertioncompounds or alloy compounds, a lithium counterelectrode and a lithiumreference electrode. Starting from the rest potential, the potential islowered at a rate of 0.01-1 mV/s to 0 V against Li/Li⁺and then movedback to the rest potential.

The charge and discharge, capacities Q_(c) and Q_(d) respectively aregiven by numerical integration of the l(t) curves obtained. The cyclingyield is obtained from the quotient Q_(c)/Q_(d).

Electrolytes which can be used are solutions of LiPF₆, LiBF₄, LiCLO₄,LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂ or LiC(CF₃SO₂)₃ and mixtures thereof, inaprotic solvents, such as EC, DMC, PC, DEC, BC, VC, cyclopentanone,sulfolane, DMS, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, EMC,MPC, BMC, EPC, BEC, DPC, 1,2-diethoxymethane, THF,2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetateand mixtures thereof. The electrolytes can also comprise organicisocyanates (DE 199 44 603) for reducing the water content. Lithiumcomplex salts of the formula

where

-   -   R¹ and R² are identical or different, are optionally directly        bonded to one another via a single or double bond, and are each,        individually or together, an aromatic ring from the group        consisting of phenyl, naphthyl, anthracenyl and phenanthrenyl,        which may be unsubstituted or mono- to hexasubstituted by alkyl        (C₁ to C₆), alkoxy groups (C₁ to C₆) or halogen (F, Cl or Br),    -   or are each, individually or together, an aromatic heterocyclic        ring from the group consisting of pyridyl, pyrazyl and        pyrimidyl, which may be unsubstituted or mono- to        tetrasubstituted by alkyl (C₁ to C₆), alkoxy groups (C₁ to C₆)        or halogen (F, Cl or Br),    -   or are each, individually or together, an aromatic ring from the        group consisting of hydrooxybenzocarboxyl        hydroxynaphthalenesulfonyl, which may be unsubstituted or mono-        to tetrasubstituted by alkyl (C₁ to C₆), alkoxy groups (C₁ to        C₆) or halogen (F, Cl of Br),    -   R³-R⁶ can each, individually or in pairs and optionally bonded        directly to one another by a single or double bond, have the        following meanings:    -   1. alkyl (C₁ to C₆), alkoxy (C₁ to C₆) or halogen (F, Cl or Br)    -   2. an aromatic ring from the groups    -   phenyl, naphthyl, anthracenyl and phenanthrenyl, which may be        unsubstituted or mono- to hexasubstituted by alkyl (C₁ to C₆)        alkoxy groups (C₁ to C₆) or halogen (F, Cl or Br),    -   pyridyl, pyrazyl and pyrimidyl, which may be unsubstituted or        mono- to tetrasubstituted by alkyl (C₁ to C₆), alkoxy -groups        (C₁ to C₆) or halogen (F, Cl or Br),    -   which are prepared by the following process (DE 199 32 317):    -   a) chlorosulfonic acid is added to 3-, 4-, 5- or 6-substituted        phenol (III) in a suitable solvent,    -   b) the intermediate (IV) from a) is reacted with        chlorotrimethylsilane, and the product is filtered and subjected        to fractional distillation,    -   c) the intermediate (II) from b) is reacted with lithium        tetramethoxyborate (1-) in a suitable solvent, and the end        product (I) is isolated therefrom,    -   can also be present in the electrolyte.

The electrolytes can likewise comprise compounds of the followingformula (DE 199 41 566):

 [([R¹(CR²R³)_(k)]_(l)A_(x))_(y)Kt]⁺⁻N(CF₃)₂

where

-   -   Kt=N, P, As, Sb, S or Se,    -   A=N, P, P(O) O, S, S(O), SO₂, As, As(O) Sb or Sb(O)    -   R¹, R², R³ are identical or different    -   and are H, halogen, substituted and/or unsubstituted alkyl        C_(n)H_(2n+1), substituted and/or unsubstituted alkenyl having        1-18 carbon atoms and one or more double bonds, substituted        and/or unsubstituted alkynyl having 1-18 carbon atoms and one or        more triple bonds, substituted and/or unsubstituted cycloalkyl        C_(m)H_(2m−1), mono- or polysubstituted and/or unsubstituted        phenyl, substituted and/or unsubstituted heteroaryl,    -   A can be included in R¹, R² and/or R³ in various positions,    -   Kt can be included in a cyclic or heterocyclic ring,    -   the groups bonded to Kt may be identical or different,        where

-   n=1-18

-   m=3-7

-   k=0 or 1-6

-   l=1 or 2 in the case where x=1 and 1 in the case where x=0

-   x=0 or 1

-   y=1-4.

However, use can also be made of electrolytes comprising compounds ofthe general formula (DE 199 53 638)X—(CYZ)_(m)—SO₂N(CR¹R²R³)₂where

-   -   X is H, F, Cl, C_(n)F_(2n−1), C_(n)F_(2n−1) or        (SO₂)_(k)N(CR¹,R², R³)₂,    -   Y is H, F or Cl    -   Z is H, F or Cl    -   R¹, R² and R³ are H and/or alkyl, fluoroalkyl or cyclo-alkyl    -   m is 0-9 and, if X=H, m≠0    -   n is 1-9    -   k is 0 if m=0 and k=1 if m=1-9,    -   and complex salts of the general formula (DE 199 51 804)        M^(x−)[EZ]^(y−) _(x/y)        in which    -   x and y are 1, 2, 3, 4, 5 or 6    -   M^(x−)is a metal ion    -   E is a Lewis acid selected from the group consisting of    -   BR¹R²R³, AlR¹R²R³, PR¹R²R³R⁴R⁵, AsR¹R²R³R⁴R⁵ and VR¹R²R³R⁴R⁵,    -   R¹ to R⁵ are identical or different, are optionally bonded        directly to one another by a single or double bond, and each,        individually or together, has the following meaning:    -   a halogen (F, Cl or Br),    -   an alkyl or alkoxy radical (C₁ to C₈), which can be partially or        fully substituted by F, Cl or Br,    -   an aromatic ring, optionally bonded via oxygen, from the group        consisting of phenyl, naphthyl, anthracenyl and phenanthrenyl,        which may be unsubstituted or mono- to hexasubstituted by alkyl        (C₁ to C₈) or F, Cl or Br,    -   an aromatic heterocyclic ring, optionally bonded via oxygen,        from the group consisting of pyridyl, pyrazyl and pyrimidyl,        which may be unsubstituted or mono- to tetrasubstituted by alkyl        (C₁ to C₈) or F, Cl or Br, and    -   Z is OR⁶, NR⁶R⁷, CR⁶R⁷R⁸, OSO₂R⁶, N(SO₂R⁶) (SO₂R⁷), C(SO₂R⁶)        (SO₂R⁷) (SO₂R⁸) or OCOR⁶, where    -   R⁶ to R⁸ are identical or different, are optionally bonded        directly to one another by a single or double bond and are each,        individually or together,    -   hydrogen or are as defined for R¹ to R⁵.

These electrolytes can be employed in electrochemical cells containingcathodes made from customary lithium intercalation and insertioncompounds, but also containing cathode materials consisting of lithiummixed oxide particles coated with one or more metal oxides (DE 199 22522) or polymers (DE 199 46 066).

0% for the control and from 0.1 to 10% (based on the total weight ofconductive salt) of additives from the group consisting of organicalkali metal salts are added. Particular preference is given toadditives from the group consisting of organic alkali metal borates andalkali metal alkoxides or tetraalkylammonium borates and alkoxides. From0.1 to 7% of additives are preferably added to the conductive salt.

On evaluation of the measurement curves, it becomes clear that theadditive decomposes by reduction at potentials of about 900-1000 mVagainst Li/Li⁺.

Through the reduction of the additive, more capacity, compared withconventional systems, is consumed in the 1st cycle. At the latest afterthe 3rd cycle, however, significantly higher cycling yields are obtainedthan without additive.

b) Behaviour of the Additives at High Potentials

In each case, 3-5 cyclic voltammograms were recorded successively in ameasurement cell containing a stainless-steel, platinum or gold workingelectrode, a lithium counterelectrode and a lithium reference electrode.To this end, starting from the rest potential, the potential was firstlyincreased at a rate of from 1 mV/s to 100 mV/s to voltages above therespective decomposition potential of the corresponding additive againstLi/Li⁺, and then moved back to the rest potential.

Depending on the oxidation potential, the additives are oxidized in thefirst cycle at potentials of from 3 V to 5 V against Li/Li⁻. However,this, oxidation does not result in a lasting current increase, as inconventional salts, such as LiPF₆, Li imide or Li methanide, but, afterpassing through a maximum with relatively low currents, results in theformation of a passivating coating layer on the working electrode.

Electrolytes which can be used are solutions of LiPF₆, LiBF₄, LiClO₄,LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂ or LiC(CF₃SO₂)₃, and mixtures thereof, inaprotic solvents, such as EC, DMC, PC, DEC, BC, VC, cyclopentanone,sulfolane, DMS, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, EMC,MPC, BMC, EPC, BEC, DPC, 1,2-diethoxymethane, THF,2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetateand mixtures thereof. 0% for the control and from 0.1 to 10% (based onthe total weight of conductive salt) of additives from the groupconsisting of organic alkali metal salts are added. Particularpreference is given to additives from the group consisting of organicalkali metal borates and alkali metal alkoxides. From 0.1 to 7% ofadditives are particularly preferably added to the conductive salt.

c) Properties of the Coating Layer Formed by Oxidation

In each case, 3-5 cyclic voltammograms are recorded successively in ameasurement cell containing a stainless-steel working electrode, alithium counterelectrode and a lithium reference electrode. Startingfrom the rest potential, the potential is firstly increased at a rate of10 mV/s-20 mV/s to values above the respective decomposition potentialof the corresponding additives. The coating layer is deposited on theelectrode in the process. The potential is then lowered to values below0 V against Li/Li⁺, thus initiating lithium deposition on thestainless-steel electrode. To this end, lithium ions must migratethrough the film formed. In order to exclude dissolution of the coatinglayer during this process, the potential is again increased to valuesabove the respective decomposition potential of the salts mentioned.Lithium cycling (evident from deposition and dissolution peaks at lowpotentials) is possible in the electrolyte. Furthermore, the coatinglayer is not dissolved by the selected process, since otherwiseoxidation of the salt used at the above-mentioned potentials would haveto be detectable in the second and all subsequent cycles.

d) Application of the Coating Layer to Certain Cathode Materials

LiMn₂O₄, LiCoO₂, LINiO₂ and LiNi_(x)Co_(1−x)O₂ cathodes are particularlyinteresting. In a measurement cell, a working electrode having one ofthe compositions indicated here, a lithium counterelectrode and alithium reference electrode are used.

Electrolytes which can be used are solutions of LiPF₆, LiBF₄, LiClO₄,LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₂ or LIC(CF₃SO₂)₃, and mixtures thereof, inaprotic solvents, such as EC, DMC, PC, DEC, BC, VC, cyclopentanone,sulfolane, DMS, 3-methyl-1,3-oxazolidin-2-one, γ-butyrolactone, EMC,MPC, BMC, EPC, BEC, DPC, 1,2-diethoxymethanre, THE,2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetateand mixtures thereof. 0% for the control and from 0.1 to 10% (based onthe total weight of conductive salt) of additives from the groupconsisting of organic alkali metal salts are added. Particularpreference is given to additives from the group consisting or organicalkali metal borates and alkali metal alkoxides. From 0.1 to 7% ofadditives are preferably added to the conductive salt.

Starting from the rest potential, the cathodes are first fully chargedagainst Li/Li⁻.

The cathode is subsequently overcharged. During this operation, thevoltage reaches an upper value determined by the measurementarrangement. If the potentiostat/galvanostat is then switched off, thepotential drops very rapidly.

Compared with the reference, the increase in potential in the range4.3-6 V against Li/Li⁺is slower in the case of the additive-containingelectrolytes according to the invention.

After the potentiostat/galvanostat has been switched off (release of thedisconnector is simulated), the potential of the cathode drops rapidlyin both electrolytes.

In the case of the electrolyte without additive, the potentialoscillates at values around 4.2-4.3 V against Li/Li⁺. Accordingly, thecathode remains in the charged, high-energy state.

By contrast, the addition of additives causes a lowering of thepotential. The potential then corresponds to the rest potential of anuncharged electrode. This suggests that additives or the film formed bydecomposition of the-additive is capable of dissipating the “excesspotential” in a controlled manner by internal self-discharge and thusconverting the battery into a low-energy state after release of thesafety means (for example disconnector).

Particularly suitable additives according to the invention are compoundsof the following formula:Li⁺B⁻(OR¹)_(m)(OR²)_(P)  (I)in which

-   -   m and p are 0, 1, 2, 3 or 4, where m+p=4, and    -   R¹ and R² are identical or different,    -   are optionally bonded directly to one another by a single or        double bond,    -   and are each, individually or together, an aromatic or aliphatic        carboxylic or sulfonic acid, or    -   are each, individually or together, an aromatic ring from the        group consisting of phenyl, naphthyl, anthracenyl and        phenanthrenyl, which may be unsubstituted or mono- to        tetrasubstituted by A or Hal, or,    -   are each, individually or together, a heterocyclic aromatic ring        from the group consisting of pyridyl, pyrazyl and bipyridyl,        which may be unsubstituted or mono- to trisubstituted by A or        Hal, or    -   are each, individually or together, an aromatic hydroxy acid        from the group consisting of aromatic hydroxycarboxylic acids        and aromatic hydroxysulfonic acids, which may be unsubstituted        or mono- to tetrasubstituted by A or Hal,    -   and    -   Hal is F, Cl or Br    -   and    -   A is alkyl having 1 to 6 carbon atoms, which may be mono- to        trihalogenated.

Particularly suitable compounds are also those of the following formula:Li⁺OR⁻  (II)in which R

-   -   is an aromatic or aliphatic carboxylic or sulfonic acid, or    -   is an aromatic ring from the group consisting of phenyl,        naphthyl, anthracenyl and phenanthrenyl, which may be        unsubstituted or mono- to tetrasubstituted by A or Hal, or,    -   is an aromatic heterocyclic ring from the group consisting of        pyridyl, pyrazyl and bipyridyl, which may be unsubstituted or        mono- to trisubstituted by A or Hal, or,    -   is an aromatic hydroxy acid from the group consisting of        aromatic hydroxycarboxylic acids and aromatic hydroxysulfonic        acids, which may be unsubstituted or mono- to tetrasubstituted        by A or Hal,    -   and    -   Hal is F, Cl or Br    -   and    -   A is alkyl having 1 to 6 carbon atoms, which may be mono- to        trihalogenated.

Particularly preferred additives are lithiumbis[1,2-benzenediolato(2-)O,O′]borate(1-), lithiumbis[3-fluoro-1,2-benzenediolato(2-)O,O′]borate(1-), lithiumbis[2,3-naphthalenediolato(2-)O,O′]borate(1-), lithiumbis[2,2′-biphenyldiolato(2-)O,O′]borate(1-), lithiumbis[salicylato(2-)O,O′]borate(1-), lithium bis[2-olatobenzenesulfonato(2-)O,O′]borate(1-), lithiumbis[5-fluoro-2-olatobenzenesulfonato(2-)O,O′]borate, lithium phenoxideand lithium 2,2-biphenoxide.

Suitable additives according to the invention are also compounds of theformula (III), which exhibit similar properties to the compounds of theformulae (I) and (II):[NR′_(w)R″_(x)R′″_(y)R″″_(z)]⁺A⁻  (III)where

-   -   w, x, y and z can be 0, 1, 2, 3 or 4, where w+x+y+z=4, and    -   R′_(w), R″_(x), R′″_(y) and R″″_(z) are identical or different        and are each alkyl having 1 to 8 carbon atoms which may in each        case be mono- to trihalogenated, and    -   A⁻is OR¹ or B(OR¹)_(m)(OR²)_(p), in which    -   m and p are 0, 1, 2, 3 or 4, where m+p=4, and    -   R¹ and R² are identical or different,    -   are optionally bonded directly to one another by a single or        double bond,    -   and are each, individually or together, an aromatic or aliphatic        carboxylic, dicarboxylic or sulfonic acid radical, or    -   are each, individually or together, an aromatic ring from the        group consisting of phenyl, naphthyl, anthracenyl and        phenanthrenyl, which may be unsubstituted or mono- to        tetrasubstituted by A or Hal, or,    -   are each, individually or together, a heterocyclic aromatic ring        from the group consisting of pyridyl, pyrazyl and bipyridyl,        which may be unsubstituted or mono- to trisubstituted by A or        Hal, or    -   are each, individually or together, an aromatic hydroxy acid        from the group consisting of aromatic hydroxycarboxylic acids        and aromatic hydroxysulfonic acids, which may be unsubstituted        or mono- to tetrasubstituted by A or Hal,    -   and    -   Hal is F, Cl or Br    -   and    -   A is alkyl having 1 to 6 carbon atoms, which may in each case be        mono- to trihalogenated.

Suitable additives according to the invention are also compounds of thefollowing formula:Z⁺P⁻(OR¹)_(m)(OR²)_(p)(OR³)_(q)  (IV)where

-   -   Z⁺is Li⁺or [NR′_(w)R″_(x)R′″_(y)R″″_(z)]⁺, where    -   w, x, y and z can be 0, 1, 2, 3 or 4, where w+x+y+z=4, and    -   R′_(w), R″_(x), R′″_(y) and R″″_(z) are identical or different        and are each alkyl having 1 to 8 carbon atoms which can in each        case be mono- to trihalogenated, and    -   where    -   m, p and q are 0, 1, 2, 3, 4, 5 or 6, where m+p+q=6, and    -   R¹, R² and R³ are identical or different,    -   are optionally bonded directly to one another by a single or        double bond,    -   and are each, individually or together, an aromatic or aliphatic        carboxylic, dicarboxylic or sulfonic acid radical, or    -   are each, individually or together, an aromatic ring from the        group consisting of phenyl, naphthyl, anthracenyl and        phenanthrenyl, which may be unsubstituted or mono- to        tetrasubstituted by A or Hal, or,    -   are each, individually or together, a heterocyclic aromatic ring        from the group consisting of pyridyl, pyrazyl and bipyridyl,        which may be unsubstituted or mono- to trisubstituted by A or        Hal, or    -   are each, individually or together, an aromatic hydroxy acid        from the group consisting of aromatic hydroxycarboxylic acids        and aromatic hydroxysulfonic acids, which may be unsubstituted        or mono- to tetrasubstituted by A or Hal,    -   and    -   Hal is F, Cl or Br    -   and    -   A is alkyl having 1 to 6 carbon atoms, which may be mono- to        trihalogenated.

The following examples are intended to illustrate the invention ingreater detail, but without representing a limitation.

EXAMPLES Example 1

In each case, 3 cyclic voltammograms were recorded successively in ameasurement cell containing a graphite anode (SFG 44 with PVDF binder),a lithium counterelectrode and a lithium reference electrode. To thisend, starting from the rest potential, the potential Ys firstly loweredat a rate of 0.1 mV/s to 0 V against Li/Li⁺ and then moved back to therest potential.

The electrolytes used were solutions of LiPF₆ in EC/DMC (1:1) containing0% (control), 1% and 5% (based on the weight of LiPF₆) of lithiumbis[salicylato(2-) -O,O′]borate(1-) (abbreviated to lithium salborate).

The results are shown in Table 1 and in FIGS. 1, 2 and 3.

TABLE 1 Cycling yields on graphite Yield Yield Electrolyte 1st cycle 3rdcycle 1 M LiPF₆ in EC/DMC (1:1) 71.7% 90.5% 1 M LiPF₆ in EC/DMC (1:1) +1% 69.5% 95.5% lithium bis[salicylato(2-)O,O′]- borate(1-) 1 M LiPF₆ inEC/DMC (1:1) + 5% 61.3% 95.1% lithium bis[salicylato(2-)O,O′]-borate(1-)

It is clearly evident from FIGS. 1 and 2 that the additive decomposesjust before the film formation by ethylene carbonate. The reductionpotential can be given as about 900-1000 mV against Li/Li⁺.

The reduction of the additive causes a somewhat greater consumption ofcapacity in the first cycle. This disadvantage is compensated for fromthe third cycle. Significantly higher cycling yields are obtained.

Example 2

Passivation of the Cathode

Lithium salts have been found which participate actively in the build-upof a passivating coating layer on the cathode. The coating layer formedis permeable to lithium ions.

TABLE 2 Selected lithium salts E_(ox) vs. Anion Li/Li⁺ [V]Bis[1,2-benzenediolato(2-)O,O′]- 3.6 borate(1-)

Bis[3-fluoro-1,2-benzenediolato(2-)- 3.75 O,O′]borate(1-)

Bis[2,3-naphthalenediolato(2-)O,O′]- 3.8 borate(1-)

Bis[2,2′-biphenyldiolato(2-)O,O′]- 4.1 borate(1-)

Bis[salicylato(2-)O,O′]borate(1-) 4.5

Bis[2-olatobenzenesulfonato(2-)- 4.3 O,O′]borate(1-)

Bis[5-fluoro-2-olatobenzene- 4.5 sulfonato(2-)O,O′]borate

Phenoxide 3.5

2,2-Biphenoxide 3.7

2a) Experiments on Platinum Electrodes

In each case, 5 cyclic voltammograms were recorded successively in ameasurement cell containing a stainless-steel, platinum or gold workingelectrode, a lithium counterelectrode and a lithium reference electrode.To this end, starting from the rest potential, the potential was firstlyincreased at a rate of 10 mV/s or 20 mV/s to 5 V against Li/Li⁺and thenmoved back to the rest potential.

The salts shown in Table 2 exhibit the following characteristicbehaviour. Depending on the oxidation potential, the salts mentioned areoxidized at potentials between 3.5 and 4.5 V against Li/Li⁺. However,this oxidation does not result in a lasting current increase, as inother salts, such as LiPF₆, Li imide or Li methanide, but, after passingthrough a maximum with relatively low currents, results in the formationof a passivating coating layer on the working electrode. FIG. 4 showsthis using the example of lithiumbis[2-olatobenzenesulfonato(2-)O,O′]borate(1-).

2b) Properties Of The Coating Layer Formed

In each case, 5 cyclic voltammograms were recorded successively in ameasurement cell containing a stainless-steel working electrode, aLithium counterelectrode and a lithium reference electrode. To this end,starting from the rest potential, the potential was firstly increased ata rate of 10 mV/s-20 mV/s to values above the respective decompositionpotential of the salts mentioned. A coating layer was deposited on theelectrode in the process. The potential was then lowered to values below0 V against Li/Li⁺, initiating deposition of lithium on thestainless-steel electrode. To this end, lithium ions must migratethrough the film formed. In order to exclude dissolution of the coatinglayer during this operation, the potential was again increased to valuesabove the respective decomposition potential of the salts mentioned.FIG. 5 shows, in representative terms, the results obtained for, lithiumbis[2-olatobenzenesulfonato(2-)O,O′]borate(1-). Lithium cycling ispossible in the electrolyte. This is evident from the deposition anddissolution peaks at low potentials. Furthermore, the coating layer isnot dissolved by the selected process, since otherwise oxidation of thesalt used at the abovementioned potentials would have to be detectablein the second and all subsequent cycles.

Example 3

Cycling experiments are carried out in button cells containing ametallic lithium anode and LiCoO₂. The electrolytes used were solutionsof LiPF₆ in EC/DMC (1:1) containing 0% (control), 1% and 5% (based onthe weight of LiPF₆) of lithium bis[salicylato(2-) -O,O′]borate(1-).

The results are shown in Tables 3, 4 and 5.

TABLE 3 System 1 M LiPF₆ in EC/DMC (1:1) Charge capacity Dischargecapacity Cycle number [mAh/g] [mAh/g] 1 164.6 153.4 2 155.1 153.6 3155.0 153.8

TABLE 4 System 1 M LiPF₆ in EC/DMC (1:1) + 1% lithiumbis[salicylato(2-)O,O′]borate(1-) Charge capacity Discharge capacityCycle number [mAh/g] [mAh/g] 1 164.0 153.7 2 155.2 153.0 3 154.0 152.8 4153.4 152.5

TABLE 5 System 1 M LiPF₆ in EC/DMC (1:1) + 5% lithiumbis[salicylato(2-)-O,O′]borate(1-) Charge capacity Discharge capacityCycle number [mAh/g] [mAh/g] 1 163.4 149.8 2 151.0 148.9 3 149.5 148.1 4148.4 147.1

It can be seen from the values shown that addition of 1% of borate doesnot have an adverse effect on the performance of the cathode used.

Example 4

Behavior of the Additive Lithium bis[Salicylato(2-) O,O′]Borate(1-) onOvercharging

The following measurement cycle was recorded in a measurement cellcontaining a LiCoO₂ working electrode, a lithium counterelectrode and alithium reference electrode.

The electrolyte used was solutions of LiPF₆ in EC/DMC (1:1) containing0% (reference) or 1.5% of lithium bis[salicylato(2-)O,O′]borate(1-).

Starting from the rest potential, the cathode was firstly charged at acharge rate of C/15-C/18 to 4.3 V against Li/Li⁺.

The cathode was subsequently overcharged at a charge rate of C/5. Duringthis operation, the voltage reached a specified maximum value of 6 Vagainst Li/Li⁺. If the potentiostat/galvanostat is then switched off,the potential drops very rapidly.

Comparison of the Curves (FIGS. 6 and 7)

Compared with the reference, the potential increase in the region 4.3-6V against Li/Li⁺is slower in the case of the electrolyte containinglithium bis[salicylato(2-)-O,O′]borate(1-). This can be explained by adesired decomposition of the additive and the consequent formation of acoating layer.

After the external potentiostat/galvanostat has been switched off, thussimulating release of the disconnector, the potential of the cathode inboth electrolytes drops rapidly.

In the case of the electrolyte with no additive, the potentialoscillates about values of 4.2-4.3 V against Li/Li⁺. The cathode remainsin the charged, high-energy state.

By contrast, addition of lithium bis[salicylato(2-) -O,O′]borate(1-)causes the potential to drop to values of about 3.7 V against Li/Li⁻.This corresponds to the rest potential of an uncharged LiCoO₂ electrode.This suggests that lithium bis[salicylato(2-) -O,O′]borate(1-) or thefilm formed by decomposition of the additive is capable of dissipatingthe “excess potential” in a controlled manner by internal self-dischargeand thus converting the battery into a low-energy state after release ofa safety means (for example disconnector).

Example 5

Cycling experiments are carried out in button cells containing ametallic lithium anode and LiMn₂O₄. The electrolytes used are solutionsof LiPF₆ in EC/DMC (1:1) containing 0% (control) and 0.2% (based on theweight of the electrolyte) of lithiumbis[2,2′-biphenyldiolato(2-)-O,O′]borate(1-).

TABLE 6 System 1 M LiPF₆ in EC/DMC (1:1) Charge capacity Dischargecapacity Cycle number [mAh/g] [mAh/g] 1 119.8 112.5 2 111.8 111.6 3111.0 110.3 4 109.8 110.2 5 111.5 110.8 6 109.6 109.0 7 108.7 108.8 8108.7 108.8 9 109.0 108.7 10 108.1 107.7

TABLE 7 System 1 M LiPF₆ in EC/DMC (1:1) + 0.2% of lithiumbis[2,2′-biphenyldiolato(2-)O,O′]borate(1-) Charge capacity Dischargecapacity Cycle number [mAh/g] [mAh/g] 1 130.5 119.8 2 118.4 117.0 3118.6 117.4 4 117.7 116.5 5 116.5 115.5 6 116.9 114.8 7 116.4 114.4 8114.5 113.4 9 113.8 112.8 10 113.9 113.6

It can be seen from the values shown that addition of 0.2% of lithiumbis[2,2′-biphenyldiolato(2-) -O,O′]borate(1-) has a significant,positive effect on the performance of the cathode used.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. An electrolyte comprising a lithium containing inorganic conductivesalt dissolved in an aprotic solvent, wherein the electrolyte furthercomprises at least one organic alkali metal salt as an additive, whereinthe organic alkali metal salt is a lithium borate of the formula (1)Li⁺B⁻(OR¹)_(m)(OR²)_(p)  (I) in which m and p are 0,1,2,3, or 4, wherem+p=4 and R¹ and R² are identical or different, are optionally bondeddirectly to one another by a single or double bond, and are each,individually or together, an aromatic or aliphatic carboxylic,dicarboxylic or sulfonic acid, or are each, individually or together, anaromatic ring selected from the group consisting of phenyl, naphthyl,anthracenyl and phenanthrenyl, which is optionally unsubstituted ormono- to tetrasubstituted by A or Hal, or are each, individually ortogether, a heterocyclic aromatic ring selected from the groupconsisting of pyridyl, pyrazyl and bipyridyl, which is optionallyunsubstituted or mono to trisubstituted by A or Hal, or are each,individually or together, an aromatic hydroxy acid selected from thegroup consisting of aromatic hydroxycarboxylic acids and aromatichydroxysulfonic acids, which is optionally unsubstituted or mono- totetrasubstituted by A or Hal, and Hal is F, Cl or Br and A is alkylhaving 1 to 6 carbon atoms, which may be mono- to trihalogenated,wherein the organic alkali metal salt additive is present inconcentrations of from 0.1 to 10% by weight based on the weight ofconductive salt.
 2. An electrolyte according to claim 1, wherein theorganic alkali metal salt additive is selected from the group consistingof lithium bis[1,2-benzenediolato(2-)0,0′]borate(1-), lithiumbis[3-fluoro-1,2-benzenediolato(2-)0,0′]borate( 1-), lithiumbis[2,3-naphthalenediolato(2-)0,0′]borate-(1-), lithiumbis[2,2′-biphenyldiolato(2-)0,0′]borate(1-), lithiumbis[2-olatobenzene-sulfonato(2-)0,0′]borate(1-), lithiumbis[salicylato(2-)-0,0′]-borate(1-), lithiumbis[5-fluoro-2-olatobenzenesulfonato(2-)0,0′]borate, lithiumbis[oxalato(2-)0,0′]borate and lithium bis[molonato(2-)0,0′]borate. 3.An electrochemical cell containing an electrolyte according to claim 1,further comprising an anode, a cathode, and a separator.
 4. Anelectrochemical cell, battery or secondary lithium battery whichcomprises an electrolyte according to claim
 1. 5. A secondary lithiumbattery comprising an electrolyte according to claim
 1. 6. Anelectrolyte according to claim 1, wherein the inorganic conductive saltis LiPF₆, LiBF₄, LiClO₄, LiAsF₆ or mixtures thereof.
 7. An electrolyteaccording to claim 1, wherein the aprotic solvent is ethylene carbonate(EC), dimethyl carbonate (DMC), propylene carbonate(PC), diethylcarbonate(DEC), butylene carbonate (BC), vinylene carbonate (VC),cyclopentanone, solfolane, dimethyl sulphone (DMS), 3-methyl-1,3-oxazolidin-2-one, y-butyrolactone, ethyl methylcarbonate(EMC), methylproply carbonate(MPC), butyl methyl carbonate(BMC), ethylpropyl carbonate (EPC), butyl ethyl carbonate (BEC),dipropyl carbonate (DPC), 1,2-diethoxymethane, tetrahydrofuran (THF),2-methyltetrahydrofuran, 1,3-dioxolane, methyl acetate, ethyl acetateand mixtures thereof.
 8. An electrolyte according to claim 1, whereinthe organic alkali metal salt additive is present in concentrations offrom 0.1 to 7% by weight based on the weight of conductive salt.
 9. Anelectrolyte according to claim 1, wherein the organic alkali metal saltadditive is present in concentrations of from 1 to 5% by weight based onthe weight of conductive salt.